Yong-Suk Suh

A Direct Assessment Approach for Structural Strength Evaluation of Cargo Containment System Under Sloshing Inside LNGC Tanks Based on Fluid Structure Interaction

The sloshing phenomenon is one of the most important and challenging issues for the design of cargo containment systems of LNG carriers and is being studied by various research groups in many countries. In this paper, a direct assessment approach based on fluid structure interaction is proposed to assess the structural safety of cargo containment systems against extreme sloshing loads. In the course of developing the methodology, two technical issues are mainly dealt with; (1) how to efficiently apply a numerical method for sloshing phenomena and (2) how to verify the structural safety criteria of cargo containment system against sloshing. The developed methodology is applied to a membrane-type cargo containment system of a large LNG carrier built in Samsung Shipyard.Copyright

A Study and Design on Tank Container for Fuel Tank of LNG Fueled Ship

The objective of this study is to investigate tank container to be used as fuel tank for LNG fueled ship. Feasibility of tank container to the fuel tank of LNG fueled ship is addressed and the advantage of tank container as fuel tank of ship is investigated. Conceptual configuration of the tank container is designed as well as structural analyses based on finite element method are carried out to meet the design regulation suggested by shipping register. Static loading is considered by structural analysis and impact test is performed. It is necessary to require SRS(shock response spectrum) in order to investigate structural safety which can meet.

Numerical Study on Designing Truncated Mooring Lines for FPSO Stability Analysis

In this paper, a numerical analysis for an internal turret moored vessel located at a 400-m water depth is conducted. The target vessel has an internal turret that is located at the 0.2 Lpp position from the fore-side, with 3 × 4 complex mooring lines installed around the turret circumference. To investigate the motion response of the vessel and the structural reliability of the lines, model tests were conducted. The KRISO ocean basin has a water depth of 3.2 m, which represents 192m using a scaling of 1:60. In order to precisely represent the real-scale condition, equivalent mooring lines needed to be designed. Truncated mooring lines were designed to supplement the restriction of the flumes water depth and increase the reliability of the model testing. These truncated mooring lines were composed of two different chains in order to match the pre-tension, simultaneously restoring the curve and variation in the effecti ve line tension. The static similarities were compared using a static pull-out test and free decaying test, and the dynamic similarities were matched via a regular wave test and combined environments test. Consequently, the designed truncated mooring system could represent the prototype mooring system relatively well in the aspects of kinematics and dynamics.

A parametric study based on spectral fatigue analysis for 170k LNGC

ABSTRACT The Spectral Fatigue Analysis is representative fatigue life assessment method for vessels. This Analysis is performed generally for the whole vessel and many assessment sites. The spectral fatigue analysis is performed through the process of hydrodynamic response analysis, global structural analysis, local structural analysis and calculation of fatigue damage. In these processes, fatigue damage is affected by many variables. The representative variables are S-N curve data, wave scatter data, wave spectrum, bandwidth effect and etc. In this paper, the effects of these variables to the fatigue damage are analyzed through the spectral fatigue analysis for 170k LNGC.

Experimental Study on Fatigue Strength of Welded Joints Under Storm Loading

Welded joints are important for fatigue strength evaluation of ship and offshore structures. However, current techniques for fatigue evaluation of welded joint under variable load is not accurate enough. Also, it cannot consider the effect of load history which is one of the important features for the variable loads. Therefore, many experimental attempts are conducted for storm model to consider the variable loading. However, studies of storm loading usually ignore the effect of calm sea loading which constitute a large portion of the marine phenomena. Because it has been believed that the contribution of calm sea loading is not dominant for fatigue life in storm loading. In this paper, fatigue tests are conducted for the specimens with transverse attachment made of high tensile steel under variable amplitude axial loading based on storm model. Considered loadings include repeated single storm, 6 or 8 kind storms sequenced randomly, and storms including calm sea condition while the mean stress and the maximum stress of loadings are changed. Moreover, the effect of three variables are investigated; 1) root mean square (RMS) value of stress amplitude, 2) mean stress shift and 3) maximum stress which can characterize the storm loading on fatigue life. In addition, experiments with calm sea loading are conducted and the effect of calm sea loading is also investigated. The storm and calm sea loadings are generated from IACS-34 wave scatter diagram. 5% strain drop criteria is introduced to define crack initiation life. Experimental results including the test results from previous study are evaluated and compared with DNV-CN 30.7 (2005) and Matsuoka’s method for the estimation of crack initiation and propagation life. From the result, it is concluded that the fatigue strength under storm loading can be evaluated by RMS value of stress amplitude. And mean stress shift is more likely to relate to fatigue strength than maximum stress. The effectiveness of the calm sea loading is depend on the existence of mean stress shift. Regarding fatigue life evaluated by DNV and Matsuoka method, both of them have almost same accuracy.Copyright

Experimental Study on the Structural Behavior of Secondary Barrier of MARK-III LNG CCS

The market of LNG (Liquefied Natural Gas) carrier is remarkably expanded in the last four or five years, and lots of LNG vessels are being built in many shipyards in the world. Membrane-type MARK-III LNG CCS (Cargo Containment System) is used more and more in the construction of LNG carrier, and it has already taken considerable market share in the business of LNG vessel. No matter how many researches have been carried out on the structure of LNG CCS, most of them are mainly focused on its macroscopic behaviors, e.g. structural response of LNG CCS under sloshing load. MARK-III secondary barrier is a matter of primary concern recently, and as already known, its major function is to protect the inner hull structure from the leakage of LNG when the primary barrier of corrugated membrane fails. A closed boundary of secondary barrier for liquid tightness is mainly sustained by the boding between flexible and rigid triplexes, where the adhesive material such as epoxy green glue is applied. The thickness of adhesive glue is about 0.4mm which is extremely thin compared with those of the other structural components of MARK-III CCS. The conventional macroscopic approach hardly gives proper description about the structural behavior of secondary barrier which requires much finer representation with the resolution of glue thickness. Most recently, even though there is an example of research on the structural responses of MARK-III secondary barrier by carrying out structural analysis using microscopic approach, it still is necessary to verify the results of structural analysis base on the experimental evidences. This research deals with an experimental study on the structural behavior of the secondary barrier of MARK-III LNG CCS. The full-scale specimen of MARK-III CCS is prepared and installed in cryogenic chamber which is quite large enough to completely enclose the specimen. As the actual secondary barrier is loaded mainly with thermal loading due to cryogenic temperature and mechanical loading due to hull deformation, the specimen undergoes cryogenic temperature maintained by the chamber and mechanical loading given by the actuator of testing machine. The structural response of secondary barrier in the specimen is maintained and controlled in such a way that the response is almost the same as that of actual secondary barrier in LNG CCS. Through the intensive study on the type and size, the specimen is so designed as to sufficiently realize the structural behavior of the secondary barrier in the actual operating condition. Since the strain gauge is elaborately installed into the thin layer of adhesive glue in the secondary barrier, it is possible to measure its precise response and to capture the realistic structural behavior of the glue. FBG (Fiber Bragg Grating) sensor is also installed in the same specimen, and the structural response of the secondary barrier is measured simultaneously, which provides data of comparison to confirm the reliability of experimental results. It is necessary to verify the performance and feasibility of the strain gauge and FBG sensor prior to actual application into the specimen, because the strain gauge and FBG sensor work in extremely thin layer under cryogenic environment. Therefore, simple tensile test is carried out, and the strain gauge and FBG sensor are examined. During cooling down process, thermal loading is increased until the temperature of secondary barrier reaches −110°C, which is maintained for a few hours to stabilize the structural response of specimen. Continually, the 4-point bending test is carried out to give additional mechanical loading which is monotonously increased until the loading reaches the ultimate strength. The strain gauge and FGB sensor measure and record the strain at each testing location during the whole process of experiment. The specimen of EC (Conventional Epoxy) glue has the smallest ultimate strength, but nevertheless it has sufficient safety factor with respect to the level of loading in actual vessel, and the EHP (Epoxy Glue with the Treatment of Hot Pad) and PU (Polyurethane) glues have much more. The experiment is simulated numerically using FEM. The material data are directly obtained through various material tests. Microscopic approach is adopted, and therefore extremely fine mesh model of which element size is almost equal to glue thickness near the secondary barrier is developed, which makes it possible to represent realistic structural behavior of adhesive glue precisely. The loading and boundary conditions are carefully arranged to embody the experimental circumstances correctly. Finally, it is possible to estimate the degree of discrepancy between the results of experiment and numerical simulation, and the correlation factor can be obtained by studying the discrepancy. The correlation factor is the final result in this research, which can be applied to the structural analysis for actual secondary barrier of MARK-III LNG CCS and improve the results of the analysis.© 2009 ASME

Dropped Object Analysis Using Non-Linear Dynamic FE Analysis

For the last few decades, necessity of direct non-linear FE analysis has been increasing for the accidental events at the vessel/offshore structures. One of major areas for the accidental design, dropped object analysis using non-linear analysis is indispensable for the verification of structural safety at the design process. This paper is concerned with the methodology, conditions, and design consideration of dropped object analysis using dynamic FE analysis. By comparing the results from direct FE analyses to those from simplified energy method described in DNV-RP-C204, necessities and advantages of direct non-linear analysis can be verified.In this paper, the effect of analysis condition is investigated using parametric study. The results are influenced by the application of failure criteria according to the rule requirements, application of material properties, dropping position, condition of the object, and so on. This study can suggest appropriate determination of the methodology and condition for the dropped object analysis using direct FE analysis.Copyright

Assessing Structural Safety of Inner Hull Structure Under Cryogenic Temperature

The market of LNG (Liquefied Natural Gas) carrier is continuously in a prosperous condition, and a lot of LNG vessels are being built in many shipyards. Membrane-type MARK-III LNG CCS (Cargo Containment System) is used more and more in the construction of LNG carrier, and it has already taken considerable market share among the various LNG CCS products. This paper deals with a study on structural safety of LNG carrier whose inner hull structure is affected by cryogenic temperature of LNG. If the primary and secondary barriers are failed simultaneously, the inner hull structure comes to be in direct contact with LNG. It is well known that the cryogenic temperature exposes the inner hull structure to fatal risk of structural failure due to brittle fracture, but nevertheless it is quite difficult to find a precedent research which explains the degree of risk and severity with due consideration of the consequence caused by structural failure of inner hull. The heat transfer test has been performed using the specimen appropriate to realize test scenario while considering cryogenic liquid flow from primary and secondary barrier into inner hull structure, and at the same time, the specimen has been tested by applying proper deformation so as to examine the structural behavior of inner hull structure under cryogenic condition. The heat transfer analysis has been performed to simulate and verify the heat transfer test, and consequently it is possible to obtain actual distribution of temperature in the inner hull structure exposed to cryogenic temperature. The structural analysis has been performed to evaluate the damage of inner hull structure and as a result to assess overall decrease of hull girder strength of LNG carrier. Finally, consequence of the decrease of global strength has been discussed.Copyright

An Experimental Study on Fatigue Life Evaluation of Welded Joints under Storm Loading

In this paper, fatigue tests are conducted for the specimens with longitudinal and transverse attachment under variable amplitude axial loading based on storm model. Considered loadings include repeated single storm, 6 or 8 storms randomly, and storms including calm sea condition while the mean stress and the maximum stress of loadings are changed. The effect of three variables are investigated; root mean square(RMS) value of stress amplitude, mean stress shift and maximum stress, which can characterize storm loading on fatigue life. In addition, experiments including calm sea loading are also carried out to investigate the effect of calm sea state. Test results are evaluated and compared with DNV-CN2005 and Matsuoka`s method for the estimation of crack initiation and propagation life. To verify the validity of the criteria, the measured crack initiation lifes are compared with the specific crack length 15mm, which are calculated with beach marks.

Application of Fatigue Reliability Analysis to a Typical Upper Deck Penetration Structure of a Ship

Fatigue crack is one of the most important failure modes for hull structures and so it is imperative to verify that the calculated fatigue life is beyond the vessel service life for fatigue prone structures in design stage. However, the amount of uncertainties lying in the analysis is quite large for such kind of analysis that the analysis often fails to explain the real phenomena correctly. To manage such a risk, design fatigue factor is usually introduced and for the determination of proper level of design fatigue factor, a quantitative assessment of the fatigue reliability is very useful. In this paper, a fatigue reliability analysis for typical upper deck penetration structures of a compact drillship is presented. Possible uncertainties in inputs and analysis models have been investigated and the variation of important factors has been quantitatively estimated. The estimated failure probability of the current practice design has shown that about 3.4% cases may have fatigue crack within 20-year service life. Proper design fatigue factors for current and higher level of load uncertainties have been investigated and suggested as 2.0 to 6.0 according to the desired safety level.Copyright